Field of the Invention
The present invention relates to the use of novel compounds that are precursors of sodium alloy(s), as a negative electrode active material in a sodium-ion battery as well as a negative electrode comprising said precursor compound of sodium alloy(s), and a sodium-ion battery comprising a negative electrode of this kind.
Technological Background
Lithium batteries have become indispensable constituents in most portable electronic devices and they are widely researched for use in electric vehicles as well as in the field of energy storage. However, their future risks being compromised, on the one hand because lithium resources are limited and on the other hand because the cost of the lithium-based raw materials has almost doubled from the time when they were first used in 1991 until today, and is still rising owing to the increasing global demand for lithium-ion accumulators. Thus, although recyclable lithium batteries are beginning to be proposed, sodium-ion batteries could constitute an alternative solution of choice and replace lithium batteries, notably owing to the greater availability of the precursors of sodium in nature (earth's crust, seawater, etc.) and their low cost.
Sodium batteries generally have a cathode in which the active material is a compound capable of inserting sodium ions reversibly, an electrolyte comprising an easily dissociable sodium salt, and an anode whose active material is a sheet of sodium Na0 or of a sodium alloy, or a compound capable of inserting sodium ions reversibly at a potential lower than that of the active material of the cathode.
The various constituents of a sodium battery are selected so as to produce, at the lowest possible cost, batteries that have a high energy density, good cycling stability and safe operation.
The use of sodium in Na/S batteries that operate at a high temperature for storage of the order of a megawatt is known. Na/NiCl2 systems for electric vehicles are also known. However, these two types of batteries (ZEBRA® batteries) only operate in a high temperature range (of the order of 270-300° C.), where they have the benefit of the high conductivity of β-alumina ceramics.
Although most of the research conducted recently has focused on the design of positive electrodes for sodium-ion batteries, negative electrodes based on carbon-containing materials other than graphite have also been proposed. In fact, it is known that graphite has poor sodium insertion properties, notably on account of the fact that sodium has an ionic radius about 55% greater than that of lithium, rendering its intercalation in certain anode materials difficult. Thus, Komaba et al. [Adv. Funct. Mater., 2011, 21, 20, 3859-3867] proposed the use of “hard” carbon (carbon containing predominantly sp2 carbon atoms) as anode active material in a sodium-ion battery. Absorption of the sodium ions on the surfaces of the nanopores of “hard” carbon facilitates their insertion and makes it possible to obtain specific capacities of the order of 250 mAh/g. However, the main drawback of “hard” carbons is consumption of a portion of the current, and therefore of the sodium ions derived from the positive electrode during the first charge, resulting in the formation of a protective “passivation” layer on the negative electrode, which prevents subsequent reaction of the electrolyte on the negative electrode, into which sodium will be inserted. This phenomenon causes a decrease in the battery's energy density and a loss of the initial capacity of from 15 to 25% in the first cycle.
Very recently, Darwiche et al. [J. Am. Chem. Soc., 2012, 134, 20805-20811] showed that pure antimony Sb of micrometric size can also be used as the anode active material in a sodium-ion battery to achieve good electrochemical performance, even better than that obtained in a lithium-ion battery. In fact, the initial specific capacity in a sodium-ion battery is 600 mAh/g and it is maintained for 160 cycles, whereas in a lithium-ion battery the initial specific capacity is about 640 mAh/g and it decreases sharply after 15 cycles. However, antimony has the drawbacks of being, on the one hand, a very toxic element and, on the other hand, a non-renewable resource that will disappear owing to intensive human exploitation. In fact, exhaustion of antimony should be definitive from 2022.
Darwiche et al. [Electrochem. Comm., 2013, 32, 18-21] also described that the use of SnSb as anode active material in a sodium-ion battery makes it possible to achieve good electrochemical performance in terms of specific capacity and cycling stability (specific capacity of 525 mAh/g maintained for 125 cycles). However, antimony has the aforementioned drawbacks and tin is, like antimony, a non-renewable resource that will disappear owing to intensive human exploitation. In fact, exhaustion of tin should be definitive around 2028.